Sorbitol acetals are employed in polyolefins as nucleating agents. Nucleating agents provide improved properties to polymers, including speeded polymer crystallization and reduced haze. One nucleating agent in widespread use is 1,3-2,4 di(benzylidene) sorbitol (known as “DBS”), which is sold by Milliken & Company as Millad® 3905 brand nucleating agent. Other sorbitol acetal compounds used as nucleating agents include: (1) bis(3,4-dimethylbenzylidene) sorbitol (sold by Milliken & Company as Millad® 3988 brand nucleating agent, also known as “DMDBS”); and (2) bis(p-methylbenzylidene) sorbitol, sold by Milliken & Company as Millad® 3940 brand nucleating agent (“MDBS”).
In polymer manufacturing and operations, sorbitol acetals may be provided as an additive powder from a hopper into polyolefin processing equipment for mixing with polymer. Commercial DMDBS in powder form is shown in FIG. 1, and DMDBS crystal 9 is seen in the upper right portion of the FIG. 1.
Sorbitol acetals sometimes do not flow readily or easily from such hoppers, which is a continuing challenge for operators of polymer additive equipment. Sorbitol acetals are inherently cohesive and compressible, which contributes to operational flow problems. Flow problems may manifest themselves as bridging and plugging, which sometimes results in reduced flow, or no flow at all. This is an operational problem for polymer mixing operations.
There are at least two common industry approaches to alleviate flow problems. The first approach employs neat sorbitol acetal powder with specially designed equipment and procedures for increasing flow of the sorbitol acetal. The drawbacks of this approach include: (1) it can be expensive to design special equipment; and (2) it may not be feasible or practical to change procedures for sorbitol acetal addition in a production plant.
A second approach is to use a pre-blend which contains a chosen diacetal of sorbitol as one component and other additives at certain ratios. The pre-blends are normally provided in the form of agglomerated pellets or granules to improve flow properties. The literature that discloses this approach includes: U.S. Pat. No. 6,673,856 (Mentink), U.S. Pat. No. 6,245,843 (Kobayashi, et al.), and Korean Published Patent Application No. 2003-0049512 (“Kwun”). Operational flexibility may be sacrificed due to the fixed ratio among different additives. Pre-blends of this type may have negative effects on the optical performance of the resulting clarified polymer parts, such as undesirable white specks or flecks in finished polymeric parts of polyolefins having pre-blended sorbitol diacetals.
Kwun describes a method of solving flow and injection problems associated with sorbitol acetal nucleating agents using organic lubricants. Kwun suggests coating the sorbitol acetal containing compound with an organic material (i.e. “lubricating component”). Kwun specifically suggests employing organic lubricants such as R—COOH acids, wherein R comprises C5-C22 carbon chains. “Metal soap” type organic coatings are recommended, as the most effective coatings for this application. In one of the examples shown in the patent, a hydrophilic SiO2 grade in micron size range was used in combination with an organic lubricating agent.
What is needed in the industry is a manner of improving the flow properties of particulate sorbitol acetals without the use of undesirable pre-blends, solvents, organic lubricants, and the like. A method and composition that can be applied without the addition of cumbersome and costly mechanical equipment would be desirable. A composition or method of deploying into polymers particulate sorbitol acetal compounds in a manner to result in smooth and uninterrupted flow from hoppers would be highly desirable. A manner of achieving high quality, low haze, polymeric parts that are substantially free of blemishes or undesirable specks would be highly desirable. The invention relates to improved flow of sorbitol acetal compounds, and is further described below.